The present invention relates to color image processing for copiers, printers, and similar rendering devices. It finds particular application in conjunction with black trapping and under print processing to eliminate undesirable image artifacts associated with misregistration of color marking to black marking, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
In the operation of copiers and printers, particularly color machines, it is highly desirable to have means for processing and enhancing text and image quality Particularly in the case of single or multi-pass color printers, it is highly desirable that a digital trapping system be employed to reduce imaging problems caused by misregistration, shift and other undesirable printed image qualities. Various systems and corrective actions have been taken to prevent the occurrence of these and other undesirable image attributes, including various aspects of image trapping and spreading.
For example, U.S. Pat. No. 5,923,821 to Birnbaum et al. (“Birnbaum”) discloses a system for trapping of a dark image object on a lighter colored background. In a system using a page description language including overprint operators, black image areas (such as text, lines, graphic objects and the like) can be set to overprint colored background areas in a manner that eliminates rendered image defects including page-colored “halo” effect that can occur when the cutout background area underlying the black object is misregistered with the black object that was intended to cover the cutout area. In other systems, Birnbaum may be employed to disable the under color removal system of a colored area that would otherwise be present under the black object. Systems using black plus at least one other toner or ink color (such as but not limited to cyan, magenta and/or yellow) can usefully employ the system. The trapping system of Birnbaum may also be employed on an object-oriented basis, such that different image object types are trapped according to the system and at least one other trapping system.
Multiple color image misregistration is one example of when digital color printing problems can arise in multi-pass color print engines. For example, when black text or lines are printed on a colored background, the registration of the black text or line and colored background area may be unaligned, a white (paper color) “halo” effect on one or more sides of the text may be created, resulting in highly visible and undesirable rendered image degradation. In the case of fine print, lines or even pictorial or small graphic images, such misregistration can appear even more pronounced, as well as making text difficult to read. In that “perfect” multi-color printing registration is difficult (or might occur only by chance on even the most precise printing engines), electronic printing technologies can turn to image processing solutions to counteract misregistration problems in order to enhance image quality.
Misregistration of the color separations or planes may actually be caused by the method in which the image planes are created. Generally, due to image quality versus hardware costs tradeoffs, image quality sacrifices are often made in favor of cost control. A manner in which registration problems in the graphic arts industry have been dealt with is by graphic arts spreading and choking techniques. “Spreading” may be defined as growth of an object within the image, while “choking” may be defined as the reduction of an object within the image. In the example of text printing on a colored background, the black text is spread, while the “hollowed out” area in the solid color background is choked. The result is a slight overlap of the solid color background with the black text image, which is able to hide misregistration problems. While trapping (or the combination of spreading and choking) is not complex to implement in the graphics arts industry, a trapping implementation in the digital electronic imaging industry can be quite difficult. In electronic imaging, one implementation includes changing the page description language (PDL) representation (format) of the image.
The present invention contemplates a new and improved method for black trapping that overcomes the above-referenced problems and others.